1. Field of the Invention
The invention relates to the field of multithreshold analog to digital (A/D) converters. In particular the invention relates to a regenerative comparator providing a clocked differential output which may be utilized efficiently in such converters.
The invention particularly relates to A/D converters of two or more sections wherein a multiplicity of error amplifiers are interspersed throughout a first section thereof, one of said error amplifiers selectably providing a differential signal for quantizing and encoding in the one or more remaining sections of the converter.
The invention further relates to such a two or more section converter wherein means are provided to permit the second and more sections to track variations in both DC quantizing reference levels as well as the peak amplitude of the error signal provided to said second or more sections.
The invention may be particularly related to a multithreshold A/D converter having an absolute value device at its input to provide a signal which is always of one polarity whereby the number of threshold devices within said converter may be reduced by fifty percent for a given number of bits output by said converter.
2. Prior Art The multithreshold A/D converter has been described as " . . . by far the most straightforward and the fastest A/D converter of all." (Hermann Schmid, Electronic Analog/Digital Conversions, Van Nostrand Reinhold Company, New York, page 296.) Schmid points out that the multithreshold A/D converter operates almost instantaneously, being limited only by the delays of one comparator and a few logic gates. He notes that the total conversion time is less than 100 nsec when using conventional comparators and ordinary TTL logic gates. The primary disadvantage noted is that the multithreshold converter requires one comparator for each threshold level, except for zero. Thus in an n-bit converter there will be 2.sup.n -1 comparators. It would thus be advantageous to provide a multithreshold A/D converter which would require significantly less than 2.sup.n -1 threshold levels for an n-bit converter.
It may be noted here that conventional comparators may provide an undecisive, ambiguous output when the voltage levels being compared are extremely close in magnitude. It would thus be desirable to provide a comparator capable of absolutely differentiating extremely close voltage levels within an extremely small period of time.
The multithreshold A/D converter is frequently referred to as a parallel-cascaded A/D converter. The analog signal to be converted is applied to a parallel network of threshold comparators. The thresholds are normally established by comparison with a set of regularly graduated reference voltages derived from a multiplicity of points along a resistive ladder network. The bit obtained at the output of the converter is a logical "1" or a logical "0" depending upon whether or not the analog input signal voltage is greater than the threshold voltage point along the resistive ladder to which it is compared.
Typically, two sets of threshold comparisons are provided within the converter. The first set provides the most significant bits, MSBs, while the second provides the least significant bits, LSBs. Obviously, the second section encodes that portion of the analog input signal voltage not accounted for by the first section of the converter. Thus, the comparisons performed within the second section must be performed with respect to what remains of the input signal voltage when the voltage accounted for by the MSBs have been subtracted therefrom. Therefore the input signal must be provided to the second comparison section in a manner which subtracts that portion of the input signal which has already been encoded as a result of threshold comparison within the first section.
Linear amplifiers are typically used to interconnect the two threshold comparison sections. Such use of linear amplifiers in prior art devices has led to restriction in the speed of operation of the converter. Further, because of the susceptibility of such linear amplifiers to variations in temperature and other operating conditions, reduced reliability and accuracy in converter operation was experienced.
Variations in temperature and operating conditions have been known to adversely affect the resistive ladder networks used for deriving the various reference threshold levels. It would thus be advantageous to provide an A/D converter in which variations in both reference ladder networks and peak error amplitude signals output by linear amplifying devices were made to track in a manner so as to increase reliability and accuracy of operation.
It is therefore an objective of the present invention to provide a multithreshold A/D converter having a reduced number of thresholds for any given number of bits output by said converter thereby significantly reducing the number of devices, for example, comparators, required within the converter.
It is a further objective of the invention to provide a new and improved comparator which may be used in such multithreshold A/D converters, which comparators are capable of absolute differentiation of minute differences in voltage levels when provided sufficient time to achieve such absolute differentiation, and which can typically differentiate voltage differences of 10 nv within a time span of 10 to 50 nsec using current bipolar technology.
It is an additional objective of the invention to provide the means whereby drift in ladder threshold reference levels and in peak error signal amplitudes may be tracked so as to provide more reliable and accurate A/D conversion.